1. Field of the Invention
The present invention relates to a process for producing a peptide. More particularly, it relates to a process for producing a peptide by using a specific enzyme as a catalsyt.
2. Description of the Prior Art
Typical conventional processes for producing peptides include the azide method, the mixed acid anhydride method, the carbodiimide method, the active ester method and the acid chloride method and the like. However, various industrial problems are encountered by these conventional processes, such as racemization of the carboxyl component of the C-terminal amino acid residue occurs. Other problems include side reactions, temperature control, selection of solvent, the properties of amino protective groups and carboxyl protective groups and the effects of functional groups on the side chains of amino acids. The fragment condensation method of preparing peptides can be applied to advantage for compounds which contain glycine (the only amino acid which cannot be racemized) at the carboxyl terminal group. However, for compounds containing any other amino acid at the carboxyl terminal group the racemization cannot be prevented. In actuality, an in any peptide synthesis, the racemization problem is serious. When racemization occurs, the purity of the product is decreased and it is necessary to separate the impure isomer from the product. This is very detrimental for any industrial operation.
Among the conventional methods for forming peptide bonds, the azide method is the only method in which racemization is not much of a problem and it is for this reason that it is a desirable method. However, the azide method involves complicated operational procedures and an urea derivative is produced in a side reaction. Because of these features the azide method is undesirable from the viewpoint of yield. In addition to the various organic chemical processes, for preparing peptides, a particular peptide synthesis using the enzyme papain or chymotrypsin has been disclosed (See, for example, J. S. Fruton "Advances in Protein Chemistry", 5, Academic Press Inc. New York, N.Y. 1949).
The reactions of the method are as follows: ##STR1## A problem common to the processes of reactions (1)-(3) is that it is necessary to remove the phenylamino group from the peptide (III) under severe conditions because the phenylamino group which is bonded to the C-terminal group of the amine component (II) cannot be easily separated from the peptide, and thus some disadvantageous cleavage of the peptide chain occurs. Because of this deficiency, this mode of peptide synthesis cannot be practically used for peptide synthesis. On the other hand, reaction (4) is accompanied by transamidation and transpeptidation side reactions and thus is not practically suitable. (See, for example, R. B. Johnston et al; J. Biol. Chem., 185, 629(1950) and J. S. Fruton et al; J. Biol. Chem. 204,891(1953). In reaction (4), the primary amino group of the acid amide bonded to the terminal group of the amine component, promotes the papain catalyzed amidase reaction. Accordingly, these processes provide only a theoretical interest in showing that papain and chymotrypsin act as catalysts for the synthesis of peptide bonds in which the phenylamino group is used as the protective group for the terminal carboxyl group of the amine component.
A need, therefore, continues to exist for a method of peptide synthesis in which the variety of difficulties encountered by the prior art procedures can be overcome.